Many items are packaged today, primarily in the area of foods, where it is vitally important to maintain the proper level of moisture content within the packaged item. For example, if a popcorn kernel becomes too dry, it will not pop in the microwave oven; and if a cracker or cereal becomes too moist, it may lose its appeal to the customer. For this reason, the moisture vapor transmission rate (MVTR) of the material used to construct such packages is critical.
One of the first packaging materials attempting to solve this need was wax coated paper. Cellulosic films also were used early on. Later synthetic polymers were developed and films such as polyethylene were commonly used. As the demands grew, it was discovered that orienting polymer films improved the moisture barrier property. Today, oriented polypropylene films are very common as packaging materials.
The most common orienting methods are to stretch it in one direction (uniaxial orientation) or two directions (biaxial orientation). One "stretching" technology is called "tentering" and involves machinery which literally grabs the film and stretches it. With oriented polypropylene films, it was found that biaxial orientation provides an increase in barrier properties and yields a film with greater tensile strength in both machine and transverse directions, as well as excellent optical clarity; hence, it is the most commonly practiced technology today.
Another means of orienting films was disclosed in Great Britain, Jan. 13, 1965, U.S. Pat. No. 980,260, by Kodak Limited. This art was called "compression rolling orientation" (CRO).
Whatever the means of orientation used, once the film is created with an adequate MVTR value, it must then be made into the form and embodiment of the desired package, most usually a bag. To form the bag, one must be able to seal the film to itself. The most common means for sealing is "heat sealing".
Many kinds of machinery have been constructed for the purpose of forming the bags while simultaneously filling them with the desired content. These are typically known as vertical form fill and seal and horizontal form fill and seal machines. The reference as to their horizontal or vertical attitude is related to the attitude at which the film is introduced.
These machines typically have forming collars or bars that shape a flat piece of film into the more tubular shape of the bag, then actuate hot metal sealing jaws from an open position to a closed position, contacting the film in order to seal it into its bag-shape. It is most often very important for the outside of the film, which comes in direct contact with hot metal surface of the sealing jaws, to have a higher melting temperature than the inside of the film. The heat can transfer through the outside of the film to melt and fuse the inner, sealant side to form the seal. When the jaws reopen, the outside, which has not melted, is not stuck to the sealing jaws. Since one polymeric material cannot have two diverse melting points, a two-ply or two layer film is used.
Two-ply materials are commonly made using lamination technology, wherein a film of barrier material is laminated to a film of sealant material via any of several means. Solvent based adhesive laminations are common, as are water based adhesive laminations. Thermal, sonic or radio frequency bonding can also be utilized. In blown or cast films, coextrusion technology is common. However, these films, if unoriented, can not provide the same MVTR barrier as oriented films unless they are much thicker.
Another technology used is extrusion coating, where a sealant polymer is generally extruded onto a barrier film and adheres because it is molten. Emulsion coating of barrier films with sealant materials is also known.
All of the aforementioned methods of providing a heat sealable barrier film, except for coextrusion, are done as secondary operations. Additionally, the methods where solvents are needed introduce the problem of solvent emissions. Therefore, a continuous method is desired, wherein coextrusion provides the base material and orientation follows in a single operation.
It is the object of the present invention to provide a multi-layer heat sealable, high moisture barrier film and method and apparatus for the production of such film.
By use of the apparatus of the present invention, one or two-side heat sealable film structures are capable of being produced. The heat sealable film made from the present invention shows high moisture barrier properties, and can provide excellent gas barrier properties as well.